1. Field of the Invention
The present invention relates to gas lasers, and more particularly to a discharge unit for a gas laser, wherein a high voltage is supplied to two discharge electrodes.
2. Background Of The Invention
Excimer lasers provide high intensity laser radiation in the ultraviolet spectral range. This makes them important tools especially for medical and surgical applications as well as for other industrial applications.
Excimer lasers are gas discharge lasers that use a rare gas such as argon and a halide gas such as fluor (for example ArF excimer laser) or a gas containing a halide (for example F2) as the laser gas.
Generally, in an excimer laser a gas mixture containing the active component and other gases is steadily provided to a discharge gap between a pair of elongated electrodes inside the laser tube by means of a fan or the like. A high voltage applied between the two electrodes causes a gas discharge in said discharge gap, whereby, from the active component of the gas, short-lived excited-state molecules are generated, whose dissociation generates ultraviolet radiation constituting the laser radiation. To increase the homogeneity of the gas discharge, in present excimer lasers a pre-ionization of the laser gas by pre-ionizers is used. As the used laser gas needs to regenerate before it can be re-used, excimer lasers are generally operated in a pulsed operation mode, wherein the laser gas in the discharge gap is being steadily replaced by fresh or regenerated laser gas provided by the fan.
The discharge electrodes for an excimer laser are usually located inside the laser tube.
The housing of an excimer laser generally consists of a metal tube having openings in a cylindrical wall on the upper side thereof. An insulating plate covers the open upper side. The metal tube and one of the discharge electrodes are grounded. A high voltage is applied to the second discharge electrode via a HV duct extending through the insulating plate.
One main problem of excimer lasers, which is still not satisfactorily solved, is the contamination of the laser gas due to the corrosive effect of the active components of the laser gas on many insulating materials which are widely used as insulators, especially on materials containing carbon molecular structures, such as many plastic materials, for example TEFLON(copyright). Due to this contamination the lifetime of the laser gas is reduced, which makes a frequent exchange of the laser gas necessary. To overcome this problem, U.S. Pat. No. 4,891,818 utilizes high-purity aluminum oxide (Al2O3) as insulator, on which the corrosive effect of the active components of the laser gas is by far reduced as compared to plastic materials.
Another, even more corrosion resistant, class of materials that can be used as insulators is fluorides.
Directly related to the above problem is the problem, that exchanging of the gas and maintenance works are expensive and time-consuming. Moreover, they are hazardous activities, as the laser gases for excimer lasers are, besides their corrosive nature, highly toxic.
A further problem is to provide an excimer laser with a high pulse repetition rate. U.S. Pat. No. 5,771,258 discloses an aerodynamic chamber design for an excimer laser to provide a high pulse repetition rate. A high repetition rate also requires a high frequency voltage to be efficiently applied to the electrodes, which becomes more difficult with increasing frequency of the voltage.
The present invention may be used in conjunction with the inventions described in the patent applications identified below and which are being filed simultaneously with the present application:
All of the foregoing applications are incorporated by reference as if fully set forth herein.
One object of the present invention is to provide a laser discharge unit for a gas laser and in particular for an excimer laser, which minimizes contamination of the laser gas and thus increases the lifetime of the laser gas.
A further object of the invention is to provide a laser discharge unit for a gas laser that allows a high frequency voltage to be efficiently applied to the discharge electrodes.
Another object of the present invention is to provide a laser discharge unit for a gas laser and in particular for an excimer laser which is easy to handle and yet powerful.
The above and further objects of the invention are achieved by a laser discharge unit with an elongated electrode plate made of an electrically conductive material, such as metal, a plurality of waveguide-like coaxial high voltage ducts extending through said electrode plate and comprising respectively a central conductive core and an insulator element made of an insulator, such as a ceramic insulator, and being arranged around said core and electrically insulating said core from said electrode plate, an elongated high voltage electrode and an elongated ground electrode, said high voltage electrode being electrically connected to said cores of said ducts and said ground electrode being electrically connected to said electrode plate, wherein said ducts are arranged spaced apart from each other.
The tube of the laser is preferably electrically connected to the ground electrode, and both are preferably grounded.
The electrode plate of the discharge unit of the present invention comprises a metal plate, preferably a pure metal plate. An advantage of this is that the laser gas does not corrode the electrode plate to the same extent as it would corrode if it were made from an insulator material such as ceramic; thus contamination of the laser gas is reduced.
The electrode plate comprises a plurality of holes, through each of which one of the high voltage ducts is guided. As the electrode plate is grounded, an insulator is required between the conductive cores of the HV ducts and the electrode plate. The number of high voltage ducts (and holes) depends on the size of the laser, in particular on the length of the electrodes. For example, for a typical excimer laser, three high voltage ducts should be used. For a larger laser with longer electrodes more than three ducts should be provided. For smaller lasers with shorter electrodes only one or two ducts may be provided.
According to the invention, the conductive cores of the ducts are respectively surrounded by an insulator element. By this construction, as compared to a whole plate made of an insulator, only a small amount of insulator material is in contact with the laser gas. Thus the contamination of the laser gas is clearly reduced.
The conductive cores of the ducts and the electrode plate form a coaxial waveguide-like structure, which facilitates the effective coupling of high frequency pulses for pulsed operation mode of the laser to the high voltage electrode.
The insulator elements of the ducts are preferably made of an insulating material, such as aluminum oxide (Al2O3). Alternatively, the insulator elements can be made from a fluoride material. In this regard, the more expensive fluoride materials, which are more resistant against corrosion by the excimer laser gas, can be used since the amount of insulator material has been reduced to a minimum according to the invention.
In principle the insulator elements may have any shape. Preferably the shape of the insulator elements is such that they conically expand towards said high voltage electrode and comprise a corrugated surface, so as to increase a creepage path extending along said surface. This shape is intended to prevent surface flashover between the high voltage electrode and the grounded electrode plate.
Preferably the electrode plate carries the high voltage ducts in that the ducts are mounted to the electrode plate by a plurality of screws. Alternatively the ducts can be mounted to the tube of the laser, for example by screws or by rods or plates or the like.
The laser discharge unit preferably further comprises a sleeve enclosing the core and insulator of each duct. Each sleeve preferably includes an inner end supported by the electrode plate, and an outer free end. Each core includes an inner end connected to the high voltage electrode plate and a threaded outer free end extending beyond the free end of the sleeve. A nut or other fastening means may be screwed onto the threaded end so as to press the sleeve against the electrode plate and thereby tension the core by pulling it. As those skilled in the art will recognize, any other construction for fixing the high voltage ducts to the electrode via the core or via the insulator element is possible as well. Preferably a stud which comprises a thread at both of its ends, such as a threaded bolt, is used to connect the inner end of the core to the high voltage electrode. Alternatively to the above described construction the high voltage electrode can be carried by, or mounted to, the laser tube, for example via a plurality of plates or rods made of an insulating material that are screwed to the tube.
The core and the insulator of each duct are preferably fixed with respect to each other; this may be accomplished, for example, by providing the inner end of each core with a core ring shoulder pressed against the insulator by the tensioned core. A seal is preferably provided between the ring shoulder and the insulator. It is also possible, however, for the core to include a recess into which a ring shoulder of the insulator may be inserted. Alternatively, the core and the insulator may be fixed with respect to each other by some different construction.
The insulator element may be pressed against the electrode plate by means of the tensioned core via the core ring shoulder at the inner end of the core and an insulator ring shoulder of the insulator element. Preferably, a seal is provided between the insulator shoulder and the electrode plate.
A sealing ring also preferably surrounds each sleeve. The sealing ring should have at its outer circumference a flange that is supported by an outer rim of a hole in the tube through which the respective high voltage duct is inserted. The electrode plate may also be provided with a ring shoulder supported at an inner rim of the tube. The ring and the electrode plate may then be connected, for example, by screws. A seal is preferably provided between the electrode plate shoulder and the inner rim of the tube.
The ground electrode is preferably carried by, or mounted to, the electrode plate. Preferably a plurality of flow guides is used for this purpose. The flow guides are preferably made from sheets of metal that extend between the electrode and ground electrode in a plane perpendicular to the longitudinal axis of the electrodes. The flow guides typically comprise an upper flange, a lower flange, and a central flow-guiding portion integrally connecting the upper flange to the lower flange. The upper and lower flanges extend perpendicular to each other and to the central flow-guiding portion. The upper flange is attached to a side face of said electrode plate, and the lower flange is attached to a bottom face of the ground electrode. Preferably the central flow-guiding portion is aerodynamically profiled in order to minimize flow resistance and turbulences for maintaining a substantially laminar gas flow between the flow guides.
Alternatively, the laser tube can carry the ground electrode, for example via a plurality of plates or rods or the like made of metal or some other conductive material.
Preferably a gas-tight seal is provided between the ducts and the electrode plate. Alternatively a seal can be provided outside the laser tube, for example at the end of the ducts. For practical reasons the holes in the electrode plate and the ducts preferably have a round cross-section. In this case, the gas-tight seals are ring-shaped. The holes can just as well have a square, a rectangular, an oval, an oblong or any other cross-section. As would be apparent to those skilled in the art, the ducts and the gas-tight seals should have a corresponding shape. But ring-shaped seals have the advantage that they are easier to fabricate and to handle, more reliable, and furthermore they are cheaper than for example rectangular seals. On the other hand it is preferred that a metal seal is used. If a ring-shaped seal is used, a commercial metal seal can be used.
It is preferred that the ducts are inserted into the electrode plate respectively with a defined tolerance between the insulator element and the respective hole in the electrode plate through which the respective duct is inserted. In this manner, the ducts are held fixed in a defined position. According to an alternative embodiment of the invention this fixing can be achieved solely by fixing elements, such as bolts or screws or a tube, by which the respective duct is held in its hole in a fixed position.
The preferred discharge unit further comprises a pair of standard corona pre-ionizers, that is a pair of elongated cylindrical pre-ionizers with a conductive core and a surrounding tube-shaped insulator. The pre-ionizers extend substantially parallel along opposite sides of the electrode. The insulator of the pre-ionizers is preferably a ceramic material such as alumina. It can also be a fluoride material. Alternatively, any other kind of known pre-ionizer can be used. The pre-ionizers are not necessary for the discharge unit to work. Indeed, excimer lasers were known before the invention of pre-ionizers. Pre-ionization, however makes the gas discharge between the high voltage electrode and the ground electrode more homogeneous and thus more reliable.
A shadow plate may be mounted between the gas discharge gap and the insulator for protection of the insulator against the laser radiation and discharge light irradiated from the gas discharge gap, and against light from the pre-ionizers.
The overall construction of the laser can be such that first a laser tube is provided, and then the high voltage electrode, the ground electrode, the insulator element or elements, the high voltage conductor or conductors and the pre-ionizers are mounted to the tube, one by one. It is preferred, however, that the electrode arrangement is a pre-mounted module-type discharge unit, wherein the electrodes, the shadow plate, and the high voltage ducts are pre-mounted independently of other laser elements. The discharge unit may be mounted to the laser tube as a whole. This provides several advantages. One advantage is that the gas discharge gap between the high voltage electrode and the ground electrode can be adjusted before the discharge unit is mounted into the laser tube. This facilitates an accurate adjustment of the gas discharge gap. Furthermore the construction of the laser can be done in a more efficient manner.
The laser gas can, in the case of an excimer laser, be any excimer laser gas, such as KrF, ArF, XeF, XeBr, HgBr, HgCl, XeCl, HCl, F2, Ar2 and the like or any laser gas in case of some other gas discharge laser.
Besides the laser gas, a buffer gas comprising a mixture of Helium, Neon and/or Argon is preferably provided in the tube.